High chlorine content low attenuation optical fiber

ABSTRACT

An optical fiber having a core comprising silica and greater than 1.5 wt % chlorine and less than 0.5 wt % F, said core having a refractive index Δ 1MAX , and an inner cladding region having refractive index Δ 2MIN  surrounding the core, where Δ 1MAX &gt;Δ 2MIN .

This application is a divisional of U.S. application Ser. No.15/444,687, filed on Feb. 28, 2017, now U.S. Pat. No. 10,429,579, whichis a divisional of and claims priority from U.S. application Ser. No.14/755,469, filed on Jun. 30, 2015, now U.S. Pat. No. 9,618,592, whichclaims the benefit of priority under 35 U.S.C. § 119 of U.S. ProvisionalApplication Ser. No. 62/022,926 filed on Jul. 10, 2014, the content ofeach is relied upon and incorporated herein by reference in itsentirety.

FIELD

The present invention relates to optical fibers having high chlorinedopant levels in the core of the fiber.

TECHNICAL BACKGROUND

There is a continuing need for lower attenuation optical fibers. Lowattenuation is one of the most critical properties in optical fibers.Most optical fibers use germania (GeO2) doped silica for the core regionand pure silica for the overclad region. However, the Raleigh scatteringdue to germania doping limits low fiber attenuation to about 0.18 dB/kmfor practical fibers due to Rayleigh scattering related to dopantconcentration fluctuation. To reduce dopant concentration fluctuation,relatively high silica core fibers have been made which utilize afluorine doped cladding. These fibers sometimes include small amounts ofchlorine. However, these high silica content core optical fibers havehigh viscosity that increases the Rayleigh scattering due to highfictive temperature in the core. In addition, the fluorine (F) dopedcladding has a much lower viscosity, which results in high draw inducedstress in the core region. The high stress in the core region reducesthe glass relaxation, which increases the Rayleigh scattering loss. Inaddition, the stress effect reduces the core refractive index throughstress-optic effects, making it difficult to achieve the core refractiveindex change required for making a single mode fiber, therefore evenhigher amounts (˜2×) of fluorine doping in the cladding is required.This higher F-doping makes the silica core and F-doped cladding haveeven higher viscosity and stress differences, resulting in the fibersbeing drawn at slow speeds to achieve low attenuation.

SUMMARY

Disclosed herein are optical waveguide fibers which comprise a corecomprising silica and greater than or equal to 1.5 wt % chlorine andless than 0.6 wt % fluorine, the core having a refractive indexΔ_(1MAX), and a cladding region having a refractive index Δ_(2MIN)surrounding the core, where Δ_(1MAX)>Δ_(2MIN). The fibers disclosedherein are preferably single moded at 1550 nm. In some embodiments, thefibers disclosed herein may exhibit a 22 m cable cutoff less than orequal to 1260 nm. In some preferred embodiments, the molar ratio ofchlorine in the core to fluorine in the cladding is greater than 1, morepreferably greater than 1.5. The fibers disclosed herein preferablycontain less than 1 weight percent GeO₂, and more preferably contain noGeO₂. In some preferred embodiments, the core comprises chlorine in anamount greater than 2 weight percent, more preferably greater than 2.5weight percent, and even more preferably greater than 3 weight percent.

In some embodiments, the core region may be doped with greater than 2.5wt % chlorine with no fluorine doping in the cladding region. In yetanother embodiment, the core region is doped with greater than 3 weight% chlorine with no fluorine doping in the cladding region.

The fiber designs disclosed herein provide a core with equal or lowerviscosity than the cladding. This results in reduced stresses within thefiber and correspondingly reduced fiber attenuation, not only because ofreduction in the viscosity mismatch but also reduction in the CTE(coefficient of thermal expansion) mismatch. Modeled examples of thesefibers have attenuations of about 0.15 dB/Km at 1550 nm, even when drawnat high draw speeds.

The fiber designs disclosed herein are capable of resulting in fibershaving optical properties that are G.652 compliant, MFD greater than 8.2microns at 1310 nm, typically between 8.2 microns and 9.4 microns at1310 nm, zero dispersion wavelength, λ₀, of 1300≤λ0≤1324 nm, cablecutoff less than or equal to 1260 nm, and attenuation at 1550 nm≤0.18dB/Km, more preferred ≤0.17 dB/Km, even more preferred ≤0.16 dB/Km at1550 nm, and even more preferred ≤0.15 dB/Km at 1550 nm, and mostpreferably ≤0.14 dB/Km at 1550 nm.

The fiber designs disclosed herein also include optical fibers havingeffective area at 1550 nm of larger than 70 micron². In someembodiments, the effective area at 1550 nm of disclosed fibers is largerthan 90 micron². In other embodiments, the effective area at 1550 nm ofdisclosed fibers is larger than 110 micron². In still other embodiments,the effective area at 1550 nm of disclosed fibers is larger than 130micron². In some preferred embodiments, optical fibers having effectiveat 1550 nm area larger than 70 micron² have cable cutoff less than 1530nm. In some other preferred embodiments, optical fibers having effectivearea at 1550 nm larger than 110 micron² have cable cutoff less than 1530nm. In still other preferred embodiments, optical fibers havingeffective area at 1550 nm larger than 130 micron² have cable cutoff lessthan 1530 nm. In some other embodiments, optical fibers having effectiveat 1550 nm area between 70 micron² and 90 micron² and a bend loss at1550 nm of less than 2 dB/turn on a 20 mm diameter mandrel. In someother embodiments, optical fibers having effective at 1550 nm areabetween 70 micron² and 90 micron² and a bend loss at 1550 nm of lessthan 1 dB/turn on a 20 mm diameter mandrel. In some other embodiments,optical fibers having effective at 1550 nm area between 70 micron² and90 micron² and a bend loss at 1550 nm of less than 0.5 dB/turn on a 20mm diameter mandrel. In some other embodiments, optical fibers havingeffective at 1550 nm area between 90 micron² and 120 micron² and a bendloss at 1550 nm of less than 3 dB/turn on a 20 mm diameter mandrel. Insome other embodiments, optical fibers having effective at 1550 nm areabetween 120 micron² and 150 micron² and a bend loss at 1550 nm of lessthan 5 dB/turn on a 20 mm diameter mandrel. Also disclosed herein is amethod of making an optical fiber, the method comprising providing anoptical fiber preform having surface area greater than 10 m²/gm to afirst furnace, and chlorine doping the preform in a first chlorinedoping step comprising exposing the preform to an atmosphere containingchlorine at temperatures higher than 1300° C. The first chlorine dopingstep may comprise exposing the preform to an atmosphere comprisingSiCl₄, and said method further comprises exposing the preform toatmosphere containing water or oxygen after said first chlorine dopingstep to further activate the preform, and said method further compriseschlorine doping the preform in a second chlorine doping step in anatmosphere containing chlorine at temperatures higher than 1300° C.

Reference will now be made in detail to the present preferredembodiments, examples of which are illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a refractive index profile corresponding to an embodimentof an optical waveguide fiber as disclosed herein.

FIG. 2 shows an alternative refractive index profile corresponding to anembodiment of an optical waveguide fiber as disclosed herein.

FIG. 3 shows an alternative refractive index profile corresponding to anembodiment of an optical waveguide fiber as disclosed herein.

FIG. 4 shows an alternative refractive index profile corresponding to anembodiment of an optical waveguide fiber as disclosed herein.

FIG. 5 illustrates a method of depositing silica soot.

FIG. 6 illustrates an apparatus and method for doping and consolidatinga soot preform.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Additional features and advantages will be set forth in the detaileddescription which follows and will be apparent to those skilled in theart from the description or recognized by practicing as described in thefollowing description together with the claims and appended drawings.

Low attenuation is one of the most critical properties in opticalfibers. Optical fibers disclosed herein are valuable for use as lowattenuation optical fibers in optical fiber cables for submarine andterrestrial long haul systems.

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and waveguide fiber radius. Theradius for each segment of the refractive index profile is given by theabbreviations r₁, r₂, r₃, r₄, etc. and lower an upper case are usedinterchangeability herein (e.g., r₁ is equivalent to R₁).

Unless stated otherwise, the “relative refractive index percent” isdefined as Δ%=100×(n_(i) ²−n_(c) ²)/2n_(i) ², and as used herein n_(c)is the average refractive index of undoped silica glass. As used herein,the relative refractive index is represented by Δ and its values aregiven in units of “%”, unless otherwise specified. The terms: delta, Δ,Δ%, %Δ, delta %, % delta and percent delta may be used interchangeablyherein. In cases where the refractive index of a region is less than theaverage refractive index of undoped silica, the relative index percentis negative and is referred to as having a depressed region or depressedindex. In cases where the refractive index of a region is greater thanthe average refractive index of the cladding region, the relative indexpercent is positive. An “updopant” is herein considered to be a dopantwhich has a propensity to raise the refractive index relative to pureundoped SiO₂. A “downdopant” is herein considered to be a dopant whichhas a propensity to lower the refractive index relative to pure undopedSiO₂. Examples of updopants include GeO₂ (germania), Al₂O₃, P₂O₅, TiO₂,Cl, Br. Examples of down dopants include fluorine.

“Chromatic dispersion”, herein referred to as “dispersion” unlessotherwise noted, of a waveguide fiber is the sum of the materialdispersion, the waveguide dispersion, and the inter-modal dispersion. Inthe case of single mode waveguide fibers the inter-modal dispersion iszero. Zero dispersion wavelength is a wavelength at which the dispersionhas a value of zero. Dispersion slope is the rate of change ofdispersion with respect to wavelength.

“Effective area” is defined as:A _(eff)=2π(∫f ² r dr)²/(∫f ⁴ r dr),where the integration limits are 0 to ∞, and f is the transversecomponent of the electric field associated with light propagated in thewaveguide. As used herein, “effective area” or “A_(eff)” refers tooptical effective area at a wavelength of 1550 nm unless otherwisenoted.

The term “α-profile” refers to a relative refractive index profile,expressed in terms of Δ(r) which is in units of “%”, where r is radius,which follows the equation,Δ(r)=A(r _(o))(1−[|r−r _(o)|/(r ₁ −r _(o))]^(α)),where r_(o) is the point at which Δ(r) is maximum, r₁ is the point atwhich Δ(r) % is zero, and r is in the range r_(i)≤r≤r_(f), where Δ isdefined above, r_(i) is the initial point of the α-profile, r_(f) is thefinal point of the α-profile, and α is an exponent which is a realnumber.

The mode field diameter (MFD) is measured using the Peterman II methodwherein, 2 w=MFD, and w²=(2∫f²r dr/∫[df/dr]² r dr), the integral limitsbeing 0 to ∞.

The bend resistance of a waveguide fiber can be gauged by inducedattenuation under prescribed test conditions, for example by deployingor wrapping the fiber around a mandrel of a prescribed diameter, e.g.,by wrapping 1 turn around a either a 6 mm, 10 mm, or 20 mm or similardiameter mandrel (e.g. “1×10 mm diameter macrobend loss” or the “1×20 mmdiameter macrobend loss”) and measuring the increase in attenuation perturn.

One type of bend test is the lateral load microbend test. In thisso-called “lateral load” test (LLWM), a prescribed length of waveguidefiber is placed between two flat plates. A #70 wire mesh is attached toone of the plates. A known length of waveguide fiber is sandwichedbetween the plates and a reference attenuation is measured while theplates are pressed together with a force of 30 Newtons. A 70 Newtonforce is then applied to the plates and the increase in attenuation indB/m is measured. The increase in attenuation is the lateral loadattenuation of the waveguide in dB/m at a specified wavelength(typically within the range of 1200-1700 nm, e.g., 1310 nm or 1550 nm or1625 nm).

Another type of bend test is the wire mesh covered drum microbend test(WMCD). In this test, a 400 mm diameter aluminum drum is wrapped withwire mesh. The mesh is wrapped tightly without stretching, and shouldhave no holes, dips, or damage. Wire mesh material specification:McMaster-Carr Supply Company (Cleveland, Ohio), part number 85385T106,corrosion-resistant type 304 stainless steel woven wire cloth, mesh perlinear inch: 165×165, wire diameter: 0.0019″, width opening: 0.0041″,open area %: 44.0. A prescribed length (750 meters) of waveguide fiberis wound at 1 m/s on the wire mesh drum at 0.050 centimeter take-uppitch while applying 80 (+/−1) grams tension. The ends of the prescribedlength of fiber are taped to maintain tension and there are no fibercrossovers. The attenuation of the optical fiber is measured at aspecified wavelength (typically within the range of 1200-1700 nm, e.g.,1310 nm or 1550 nm or 1625 nm); a reference attenuation is measured onthe optical fiber wound on a smooth drum. The increase in attenuation isthe wire mesh covered drum attenuation of the waveguide in dB/km at aspecified wavelength (typically within the range of 1200-1700 nm, e.g.,1310 nm or 1550 nm or 1625 nm).

The “pin array” bend test is used to compare relative resistance ofwaveguide fiber to bending. To perform this test, attenuation loss ismeasured for a waveguide fiber with essentially no induced bending loss.The waveguide fiber is then woven about the pin array and attenuationagain measured. The loss induced by bending is the difference betweenthe two measured attenuations. The pin array is a set of ten cylindricalpins arranged in a single row and held in a fixed vertical position on aflat surface. The pin spacing is 5 mm, center to center. The pindiameter is 0.67 mm. During testing, sufficient tension is applied tomake the waveguide fiber conform to a portion of the pin surface. Theincrease in attenuation is the pin array attenuation in dB of thewaveguide at a specified wavelength (typically within the range of1200-1700 nm, e.g., 1310 nm or 1550 nm or 1625 nm).

The theoretical fiber cutoff wavelength, or “theoretical fiber cutoff”,or “theoretical cutoff”, for a given mode, is the wavelength above whichguided light cannot propagate in that mode. A mathematical definitioncan be found in Single Mode Fiber Optics, Jeunhomme, pp. 39-44, MarcelDekker, New York, 1990 wherein the theoretical fiber cutoff is describedas the wavelength at which the mode propagation constant becomes equalto the plane wave propagation constant in the outer cladding. Thistheoretical wavelength is appropriate for an infinitely long, perfectlystraight fiber that has no diameter variations.

Fiber cutoff is measured by the standard 2 m fiber cutoff test, FOTP-80(EIA-TIA-455-80), to yield the “fiber cutoff wavelength”, also known asthe “2 m fiber cutoff” or “measured cutoff”. The FOTP-80 standard testis performed to either strip out the higher order modes using acontrolled amount of bending, or to normalize the spectral response ofthe fiber to that of a multimode fiber.

By cabled cutoff wavelength, or “cabled cutoff” as used herein, we meanthe 22 m cabled cutoff test described in the EIA-445 Fiber Optic TestProcedures, which are part of the EIA-TIA Fiber Optics Standards, thatis, the Electronics Industry Alliance—Telecommunications IndustryAssociation Fiber Optics Standards.

Unless otherwise noted herein, optical properties (such as dispersion,dispersion slope, etc.) are reported for the LP01 mode.

The fibers disclosed herein preferably exhibit a 22 m cable cutoff lessthan or equal to 1530 nm, in some embodiments less than or equal to 1400nm, in some embodiments less than or equal to 1260 nm.

Modeled examples of these fibers have attenuations of about 0.165 dB/Kmor less at 1550 nm, even when drawn at high speeds. That is, draw speeds≥10 m/s, in some embodiments, ≥15 m/s, in some embodiments, ≥25 m/s, insome embodiments, ≥35 m/s and in some embodiments, ≥45 m/s.

In some embodiments, optical fibers disclosed herein may be single modedat 1550 nm and capable of exhibiting an effective area at 1550 nm whichis greater than about 55 microns², in some embodiments between 55 and150 microns², in some embodiments between about 65 and 120 microns². Insome preferred embodiments, the effective area at 1550 nm is betweenabout 70 and 95 micron. In some preferred embodiments, the opticalfibers may have an effective area at 1550 nm larger than 70 micron² andalso exhibit a cable cutoff less than 1530 nm. In some preferredembodiments, the optical fibers may have an effective area at 1550 nmlarger than 110 micron² and a cable cutoff less than 1530 nm. In somepreferred embodiments, the optical fibers disclosed herein may exhibitan effective area at 1550 nm larger than 130 micron² and a cable cutoffless than 1530 nm. In some embodiments, the optical fibers may have aneffective area at 1550 nm area between 70 micron² and 90 micron² and abend loss at 1550 nm of less than 2 dB/turn on a 20 mm diameter mandrel.In some embodiments, the optical fibers may have an effective area at1550 nm area between 70 micron² and 90 micron² and a bend loss at 1550nm of less than 1 dB/turn on a 20 mm diameter mandrel. In some otherembodiments, optical fibers may have an effective area at 1550 nm areabetween 70 micron² and 90 micron² and a bend loss at 1550 nm of lessthan 0.5 dB/turn on a 20 mm diameter mandrel. In some other embodiments,the optical fibers may have an effective area at 1550 nm area between 90micron² and 120 micron² and a bend loss at 1550 nm of less than 3dB/turn on a 20 mm diameter mandrel. In some other embodiments, theoptical fibers may have an effective area at 1550 nm area between 120micron² and 150 micron² and a bend loss at 1550 nm of less than 5dB/turn on a 20 mm diameter mandrel.

One exemplary fiber 10, shown in FIG. 1, includes a central glass coreregion 1 comprising maximum refractive index delta percent Δ_(1MAX). Afirst depressed inner cladding region 2 surrounds central core region 1,the first inner cladding region 2 comprising refractive index deltapercent Δ_(2MIN), where Δ_(1MAX)>Δ_(2MIN). Inner cladding region 2 ispreferably immediately adjacent to central core glass region 1. Glasscore region 1 comprises silica glass, greater than 1.5 wt % chlorine andless than 0.5 wt % fluorine. Glass core region 1 preferably comprisesless than 1 weight percent GeO₂, and more preferably contains no GeO₂.In some embodiments, the glass core region 1 comprises silica glassdoped with greater than 2 wt % chlorine. In some other embodiments, theglass core region 1 comprises silica glass doped with greater than 2.5wt % chlorine. In some other embodiments, the glass core region 1comprises silica glass doped with greater than 3 wt % chlorine. In stillother embodiments, the glass region 1 comprises silica glass doped withgreater than 2.5 wt % chlorine and is preferably essentially free offluorine. Inner cladding region 2 comprises silica doped with fluorine.The term Cl_(ore) represents the chlorine dopant amount (mole %) in thecore region and the term F_(inner clad) represents the fluorine dopantamount (mole %) in the inner clad region.

Central core region 1 comprises an outer radius r₁ which is defined aswhere a tangent line drawn through maximum slope of the refractive indexof central core region 1 crosses the zero delta line. In someembodiments core region 1 may comprise greater than 1.5 wt % chlorineand less than 0.6 wt % fluorine, in other embodiments greater than 2.0wt % chlorine in other embodiments greater than 2.5 wt % chlorine, andin other embodiments greater than 3.0 wt % chlorine. Core region 1 maycomprise less than 0.6 wt % fluorine, in some embodiments less than 0.5wt % fluorine, in some embodiments less than 0.25 wt % fluorine. Morepreferably, core region 1 is essentially free of fluorine, and mostpreferably core region 1 contains no fluorine. Core region 1 may bedesigned to exhibit a maximum refractive index delta percent, Δ_(1max),between about 0.15% Δ to about 0.5% Δ, and in some embodiments betweenabout 0.15% Δ to 0.3% Δ, and in other embodiments between about 0.18%4to 0.25% Δ. Core radius r₁ is between 3 and 10 microns, and in someembodiments between about 3 to 7 microns. Central core region 1 maycomprise a single segment, step index profile. In some embodiments,central core region 1 exhibits an alpha greater than 0.5 and less than200, and in some embodiments greater than 5, in some embodiments greaterthan 10, and in some embodiments greater than 10 and less than or equalto 100.

In the embodiment illustrated in FIG. 1, inner cladding region 2surrounds central core region 1 and comprises inner radius r₁ and outerradius r₂, r₁ being defined as above and r₂ being defined as where therefractive index profile curve crosses the zero delta line, unless theentire cladding is fluorine doped, in which case outer radius r₂ isequal to the outer cladding of the optical fiber. Inner cladding region2 may comprise greater than 0.15 wt % fluorine, in some embodimentsgreater than 0.25 wt % fluorine, in some embodiments greater than 0.35wt % fluorine, and in some embodiments contains less than 1 weightpercent fluorine, and in some embodiments greater than 0.35 wt % andless than 0.8 weight percent fluorine. The molar ratio of chlorine inthe core region 1 to fluorine in inner cladding region 2 is preferablygreater than 1, in some embodiments greater than 1.5, in someembodiments greater than 2, in some embodiments greater than 2.5, and insome embodiments greater than 3.0.

FIGS. 2-4 illustrate embodiments wherein the fiber comprises an outercladding region surrounding the inner cladding region, said outercladding region having average refractive index Δ₃ whereinΔ_(1MAX)>Δ₃>Δ_(2MIN). The outer cladding has a maximum relativerefractive index Δ_(3MAX). In FIG. 2, the index of refraction of theouter cladding region 3 is equal to that of undoped SiO₂, such as may beachieved by using undoped SiO₂ as the material for forming the outercladding layer 3. In FIG. 3, the index of refraction of the outercladding region 3 is less than that of undoped SiO₂, such as may beachieved by using fluorine doped SiO₂ as the material for forming theouter cladding layer. In FIG. 4, the index of refraction of the outercladding region 3 is higher than that of undoped SiO₂, such as may beachieved by using SiON doped or chlorine doped SiO₂ as the material forforming the outer cladding layer. Thus, as described above, the outercladding region may be comprised of SiO₂ or SiON. In each of theseembodiments set forth in FIGS. 2-4, the inner cladding region 2 mayexhibit a width (r₂−r₁) between about 30 to 52 microns, in someembodiments 40 to 52 microns, and in some embodiments between about 45to 52 microns. In some embodiments, R₂ may be greater than 40, greaterthan 45 microns, or greater than 50 microns and less or equal to than62.5 microns, in some embodiments less than or equal to 56 microns orless than or equal to 51 microns.

In FIG. 4, outer cladding region 3 comprises a higher refractive indexthan inner cladding region 2, and preferably comprises refractive indexdelta percent Δ₃ which is greater than 0.002, preferably at least 0.005,for example at least 0.01, and may be greater than 0.02 or 0.03 percentdelta. Preferably, the higher index portion (compared to inner claddingregion 2) of outer cladding region 3 extends at least to the point wherethe optical power which would be transmitted through the optical fiberis greater than or equal to 90% of the optical power transmitted, morepreferably to the point where the optical power which would betransmitted through the optical fiber is greater than or equal to 95% ofthe optical power transmitted, and most preferably to the point wherethe optical power which would be transmitted through the optical fiberis greater than or equal to 98% of the optical power transmitted. Inmany embodiments, this is achieved by having the “updoped” third annularregion extend at least to a radial point of about 30 microns. In someembodiments, outer cladding region 3 comprises chlorine (Cl) in anamount greater than 200 ppm when compared to that of the inner claddingregion 2, for example greater than 400 or 700 or 1000 ppm or more, andin some embodiments preferably greater than 1500 ppm, and, in someembodiments, greater than 2000 ppm (0.2%) by weight (e.g., 2200 ppm,2500 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, 10000 ppm, or therebetween).

Various exemplary embodiments will be further clarified by the followingexamples. It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the claims.

Table 1 below illustrates modeled comparative fibers 1 and 2 as well asexemplary fibers 1-5, all of which have a core radius of 4.5 microns anda fiber diameter of 125 microns. Comparative fibers 1 and 2 do not havesignificantly high chlorine content, nor do they have a molar ratio ofchlorine in core region 1/fluorine in the inner cladding region 2 of 1or greater. Consequently, when drawn at a relatively higher drawtensions, e.g. greater than or equal to 100, 120, or 150 grams, due tothe stress optic effect a much higher amount of fluorine is needed toachieve a real refractive core to inner cladding refractive index deltaof approximately 0.34% Δ. Also set forth in Table 1 are weight percent,mole percent and modeled delta index percent of chlorine in the coreregion 1; and weight percent, mole percent and modeled delta indexpercent of fluorine in the inner cladding region 2. Also set forth arethe weight percent and mole percent ratios of chlorine in core region1/fluorine in the inner cladding region 2, as well as the expectedcore/inner cladding refractive index delta (obtained by adding theabsolute magnitude of expected delta index percent of chlorine in thecore and the expected delta index percent of fluorine in the innercladding region) and the real core/inner cladding refractive index deltaobtained by drawing the fiber at 120 grams draw tension. ComparativeExamples 1 and 2 illustrate that, for a silica core containing 1.1 wt.percent chlorine, rather than merely having to add the 0.74 weightpercent fluorine that one would expect to have to add to achieve a 0.34percent refractive index delta between the core and the fluorine dopedinner clad region, in reality because of the stress optic effect, 1.42weight percent fluorine must be added. Examples 1 through 5 listed inTable 1, all of which exhibit a molar ratio of chlorine in the coreregion 1 to fluorine in the inner cladding region 2 of greater than 1,illustrate that a much smaller amount of fluorine is needed to be addedin the inner cladding region 2 to achieve a real effective index ofabout 0.34% Δ, after being draw at 120 grams draw tension. Note that inTable 1, the core/clad index delta is the delta achieved by the chlorinedoped core region 1 relative to the fluorine doped cladding region 2,unlike the other references herein to refractive index delta.

TABLE 1 Example Dopant parameter Comparative 1 Comparative 2 1 2 3 4 5Chlorine weight % 1.1 1.1 1.8 2.0 2.5 3.0 3.4 (Cl) in core mole % 1.81.8 2.9 3.3 4.1 4.9 5.6 delta index, % 0.11 0.11 0.18 0.20 0.25 0.300.34 Fluorine (F) weight % 0.74 1.42 0.72 0.61 0.39 0.19 0.00 in cladmole % 2.3 4.3 2.2 1.9 1.2 0.6 0.0 delta index, % −0.23 −0.44 −0.22−0.19 −0.12 −0.06 0.00 Cl-core, F- Cl/F weight ratio 1.5 0.8 2.5 3.3 6.515.5 >100 Clad Cl/F mole ratio 0.8 0.4 1.3 1.7 3.5 8.3 >100 Core-clad,delta index in 0.34 0.55 0.40 0.39 0.37 0.36 0.34 preform, % Fiber drawnat 120 g 0.26 0.34 0.33 0.33 0.34 0.34 0.34 tension, delta index infiber, %

Examples of modeled chlorine doped (and Cl, GeO2 co-doped) core andfluorine doped clad optical fibers and properties are shown in Table 2.The modeled fibers in Table 2 assume a draw tension of 50 grams. Setforth in Table 2 are delta percent Δ_(1max) of the core, core alpha,core dopant, weight and mole % Cl in the core, radius R₁ of the core,delta percent Δ_(2min) of the inner cladding, inner cladding dopant,weight and mole % F in the first inner cladding, outer radius R₂ of theinner cladding, outer radius of the optical fiber R₃, ratio of (Cl incore region 1/F in first clad region 2) in wt. %/wt. % and in mole%/mole %, 22 meter cable cutoff wavelength, zero dispersion wavelength,mode field diameter at 1310 nm, effective area at 1310 nm, dispersionand dispersion slope at 1310 nm, mode field diameter at 1550 nm,effective area at 1550 nm, dispersion and dispersion slope at 1550 nm,and pin array loss, lateral load loss, and attenuation at 1550 nm. Theterm “na” refers to “not applicable” due to the cutoff wavelength beinghigh enough that the fiber is not single moded at ˜1300 nm, andconsequently properties are not reported for the 1310 operating window.

TABLE 2 Chlorine doped (and Cl, GeO2 co-doped) core and fluorine dopedclad optical fibers and properties. Parameter Example 6 Example 7Example 8 Example 9 Example 10 Example 11 Delta_(1max) (%) 0.17 0.230.17 0.17 0.2 0.17 Core alpha 100 100 100 100 100 100 Core dopant Cl ClCl Cl Cl Cl Cl in core (wt. %) 1.7 2.3 1.7 1.7 2.0 1.7 Cl in core (mole%) 2.8 3.8 2.8 2.8 3.3 2.8 R₁ (microns) 4.2 4.3 5.5 4.2 5.8 6.6Delta_(2min) (%) −0.17 −0.1 −0.1 −0.17 −0.05 −0.1 First clad dopant F FF F F F F in first clad (wt. %) 0.55 0.32 0.32 0.55 0.16 0.32 F in firstclad (mole %) 1.7 1.0 1.0 1.7 0.5 1.0 R₂ (microns) na na na 40 50 20.9Delta₃ (%) −0.17 −0.1 −0.1 0 0 −0.05 Cl in core/F in first clad (wt. 3.17.1 5.3 3.1 12.4 5.3 %/wt. %) Cl in core/F in first clad 1.6 3.8 2.8 1.66.6 2.8 (mole %/mole %) Second clad dopant F F F none none F R_(max)(microns) 62.5 62.5 62.5 62.5 62.5 62.5 22 meter cable cutoff (nm) 11381149 1350 1138 1372 1528 Zero-dispersion wavelength 1303 1305 na 1306 nana (nm) Mode field diameter @ 1310 9.1 9.2 na 9.0 na na nm (microns)Effective area @ 1310 nm 64.9 66.7 na 64.0 na na microns²) Dispersion @1310 nm 0.56 0.41 na 0.33 na na (ps/nm/km) Dispersion Slope @ 1310 nm0.0843 0.0850 na 0.840 na na (ps/nm²/km) Mode field diameter @ 1550 10.310.5 12.0 10.3 12.5 12.9 nm (microns) Effective area @ 1550 nm 81.2 83.6113.3 80.7 122.9 135.5 (microns²) Dispersion @ 1550 nm 16.9 16.9 19.816.6 19.9 21.1 (ps/nm/km) Dispersion Slope @ 1550 nm 0.0567 0.05720.0599 0.0566 0.0605 0.0618 (ps/nm²/km) Pin array @1550 nm (dB) 10.012.5 12.7 11.6 19.6 8.99 Lateral load @1550 nm (dB) 0.20 0.24 0.86 0.201.43 3.89 Attenuation at 1550 nm, 0.16 0.15 0.16 0.16 0.15 0.16 dB/kmMacrobend Loss at 1550 nm, 10.7 11.9 13.8 10.7 19.8 6.7 10 mm diametermandrel (dB/turn) Macrobend Loss at 1550 nm, 2.3 2.6 3.0 2.3 4.5 1.6 15mm diameter mandrel (dB/turn) Macrobend Loss at 1550 nm, 0.48 0.55 0.660.48 1.02 0.38 20 mm diameter mandrel (dB/turn) Macrobend Loss at 1550nm, 0.008 0.010 0.012 0.008 0.020 0.016 30 mm diameter mandrel (dB/turn)Parameter Example 12 Example 13 Example 14 Example 15 Example 16Delta_(1max) (%) 0.34 0.34 0.34 0.347 0.21 Core alpha 100 100 100 100 2Core dopant Cl Cl + GeO2 (1:1 Cl + GeO2 (1:1 Cl Cl by index) by index)Cl in core (wt. %) 3.4 1.7 1.7 3.47 2.1 Cl in core (mole %) 5.6 2.8 2.85.7 3.5 R₁ (microns) 4.2 4.45 4.9 4.95 6.0 Delta_(2min) (%) 0 0 −0.07 0−0.2 First clad dopant none none F none F F in first clad (wt. %) 0 00.23 0 0.64 F in first clad (mole %) 0 0 0.7 0 2.0 R₂ (microns) na na14.8 15.4 25.4 Delta₃ (%) 0 0 0 0.07 −0.13 Cl in core/F in first clad(wt. >100 >100 7.5 >100 3.3 %/wt. %) Cl in core/F in firstclad >100 >100 4.0 >100 1.8 (mole %/mole %) Second clad dopant none nonenone Cl F R_(max) (microns) 62.5 62.5 62.5 62.5 62.5 22 meter cablecutoff, nm 1138 1214 1388 1253 1240 Zero-dispersion wavelength 1306 13011289 1284 1305 (nm) Mode field diameter @ 1310 9.1 9.2 9.1 9.65 9.2 nm(microns) Effective area @ 1310 nm 66.2 68.0 68.6 75.9 64.7 (microns²)Dispersion @ 1310 nm 0.35 0.75 2.55 2.13 0.42 (ps/nm/km) DispersionSlope @ 1310 nm 0.0860 0.0866 0.0881 0.0828 0.0883 (ps/nm²/km) Modefield diameter @ 1550 10.3 10.4 10.0 10.7 10.5 nm (microns) Effectivearea @ 1550 nm 80.2 83.6 80.1 90.4 82.2 (microns²) Dispersion @ 1550 nm17.0 17.5 19.6 19.1 17.6 (ps/nm/km) Dispersion Slope @ 1550 nm 0.05760.0579 0.0587 0.0593 0.0597 (ps/nm²/km) Pin array @1550 nm (dB) 8.2 6.23.6 1.358 4.2 Lateral load @1550 nm (dB) 0.21 0.20 0.13 0.15 0.22Attenuation at 1550 nm, 0.16 0.17 0.17 0.16 0.16 dB/km Macrobend Loss at1550 nm, 10.7 7.1 1.4 8.3 1.5 10 mm diameter mandrel (dB/turn) MacrobendLoss at 1550 nm, 2.3 1.4 0.3 2.0 0.4 15 mm diameter mandrel (dB/turn)Macrobend Loss at 1550 nm, 0.48 0.29 0.05 0.46 0.09 20 mm diametermandrel (dB/turn) Macrobend Loss at 1550 nm, 0.008 0.005 0.001 0.0160.009 30 mm diameter mandrel (dB/turn) Parameter Example 17 Example 18Delta_(1max) (%) 0.2 0.2 Core alpha 100 100 Core dopant Cl Cl Cl in core(wt. %) 2 2 Cl in core (mole %) 3.3 3.3 R₁ (microns) 4.04 4.2Delta_(2min) (%) −0.13 −0.13 First clad dopant F F F in first clad (wt.%) 0.42 0.42 F in first clad (mole %) 1.3 1.3 R₂ (microns) 11.4 12.1 Clin core/F in first clad (wt. 4.8 4.8 %/wt. %) Cl in core/F in first clad2.5 2.5 (mole %/mole %) Delta_(2amin) (%) −0.55 −0.70 Trench dopant F FF in trench (wt. %) 1.8 2.3 F in trench (mole %) 5.5 7 R₃ (microns) 16.815.2 Trench width (microns) 5.4 3.1 Delta_(3a) (%) −0.13 −0.13 R_(3a)(microns) 16.8 21.6 Delta3 (%) −0.13 0 Second clad dopant F none R_(max)(microns) 62.5 62.5 22 meter cable cutoff, nm 1210 1254 Zero-dispersionwavelength 1306 1300 (nm) Mode field diameter @ 1310 9.0 9.2 nm(microns) Effective area @ 1310 nm 63.5 67.2 (microns²) Dispersion @1310 nm 0.11 0.95 (ps/nm/km) Dispersion Slope @ 1310 nm 0.0835 0.0875(ps/nm²/km) Mode field diameter @ 1550 10.2 10..4 nm (microns) Effectivearea @ 1550 nm 79.7 82.7 (microns²) Dispersion @ 1550 nm 17.5 18.2(ps/nm/km) Dispersion Slope @ 1550 nm 0.0609 0.0596 (ps/nm²/km) Pinarray @1550 nm (dB) 9.8 6.5 Lateral load @1550 nm (dB) 0.20 0.2Attenuation at 1550 nm, 0.17 0.17 dB/km Macrobend Loss at 1550 nm, 0.170.40 10 mm diameter mandrel (dB/turn) Macrobend Loss at 1550 nm, 0.040.09 15 mm diameter mandrel (dB/turn) Macrobend Loss at 1550 nm, 0.010.02 20 mm diameter mandrel (dB/turn) Macrobend Loss at 1550 nm, 0.0020.002 30 mm diameter mandrel (dB/turn)

Examples 6 and 7 represent optical fibers having chlorine doped stepindex core, fluorine doped cladding with the molar ratio of chlorine inthe core region 1 to the fluorine in the inner cladding region 2 greaterthan 1 (i.e., Cl/F>1 by mole) and having optical properties that arecompliant with ITU-G.652 standards, with cable cutoff less than 1260 nm,zero dispersion wavelength between 1300 nm and 1324 nm and MFD at 1310nm between 8.2 microns and 9.5 microns, and macrobend loss at 1550 nm,on a 20 mm and 30 mm diameter mandrel, respectively, of ≤1 and ≤0.01dB/turn, respectively. Example 9 represents an optical fiber havingchlorine doped step index core, fluorine doped cladding with the molarratio of chlorine in the core region 1 to the fluorine in the innercladding region 2 greater than 1 (i.e., Cl/F>1 by mole) and havingoptical properties that are compliant with ITU-G.652 standards andhaving a stress relieving outerclad silica layer starting at radialposition of 40 microns, and macrobend loss at 1550 nm, on a 20 mm and 30mm diameter mandrel, respectively, of ≤1 and ≤0.01 dB/turn,respectively. Examples 8, 10, and 11 represent optical fibers havingchlorine doped step index core, fluorine doped cladding with the molarratio of chlorine in the core region 1 to the fluorine in the innercladding region 2 greater than 1 (i.e., Cl/F>1 by mole) that are singlemoded at 1550 nm and have effective area between 110 micron and 150micron² and having optical properties that are compliant with ITU-G.654standards and macrobend loss at 1550 nm, on a 20 mm and 30 mm diametermandrel, respectively, of ≤1 and ≤0.02 dB/turn, respectively. Examples12 and 15 represent optical fibers having chlorine doped step index corein excess of 3 weight %, no fluorine doping in the cladding and havingoptical properties that are compliant with ITU-G.652 standards, withcable cutoff less than 1260 nm, zero dispersion wavelength between 1300nm and 1324 nm and MFD at 1310 nm between 8.2 microns and 9.5 micronsand macrobend loss at 1550 nm, on a 20 mm and 30 mm diameter mandrel,respectively, of ≤0.5 and ≤0.02 dB/turn, respectively. Example 13represents an optical fiber having a chlorine and GeO2 co-doped stepindex core, no fluorine doping in the cladding and having opticalproperties that are compliant with ITU-G.652 standards, with cablecutoff less than 1260 nm, zero dispersion wavelength between 1300 nm and1324 nm and MFD at 1310 nm between 8.2 microns and 9.5 microns andmacrobend loss at 1550 nm, on a 20 mm and 30 mm diameter mandrel,respectively, of ≤0.5 and ≤0.01 dB/turn, respectively. Example 14represents an optical fiber having a chlorine and GeO2 co-doped stepindex core, fluorine doping in the inner cladding and having macrobendloss at 1550 nm, on a 20 mm and 30 mm diameter mandrel, respectively, of≤0.1 and ≤0.003 dB/turn, respectively. Example 16 represents an opticalfiber having chlorine doped low alpha core profile, fluorine dopedcladding with the molar ratio of chlorine in the core region 1 to thefluorine in the inner cladding region 2 greater than 1 (i.e., Cl/F>1 bymole) and having optical properties that are compliant with ITU-G.652standards, with cable cutoff less than 1260 nm, zero dispersionwavelength between 1300 nm and 1324 nm and MFD at 1310 nm between 8.2microns and 9.5 microns, and macrobend loss at 1550 nm, on a 15 mm, 20mm and 30 mm diameter mandrel, respectively, of ≤0.5, ≤0.1 and ≤0.01dB/turn, respectively. Examples 16 and 17 represent optical fibershaving chlorine doped step index core, fluorine doped cladding with themolar ratio of chlorine in the core region 1 to the fluorine in theinner cladding region 2 greater than 1 (i.e., Cl/F>1 by mole), anfluorine doped trench offset from the core and having optical propertiesthat are compliant with ITU-G.652 standards, with cable cutoff less than1260 nm, zero dispersion wavelength between 1300 nm and 1324 nm and MFDat 1310 nm between 8.2 microns and 9.5 microns, and macrobend loss at1550 nm, on a 10 mm, 15 mm, 20 mm and 30 mm diameter mandrel,respectively, of ≤0.5, ≤0.1, ≤0.01 and ≤0.003 dB/turn, respectively.

Examples of manufactured chlorine doped core and fluorine doped cladoptical fibers and properties are shown below.

Example 1

A 1 meter long 3000 gram silica soot preform having a density of about0.5 g/cm³ was prepared in a lathe by flame depositing silica soot onto10 mm diameter removable alumina rotating bait rod comprising a silicahandle. The soot preform was placed into a consolidation furnace anddried at about 1225° C. in an atmosphere comprising about 55 volumepercent helium and about 45 volume percent SiCl4. The assembly was thentraversed (down-driven) through a hot zone having a peak temperature ofabout 1500° C. at a temperature ramp rate of about 2.5° C./min, thusproducing a fully densified Cl-doped silica glass core preform.

This preform was placed for about 24 hours in an argon purged holdingoven set at 1000° C. in order to outgas dissolved helium in the glass.The preform was then placed in a redraw furnace set at about 1900° C.,vacuum was applied through the handle to the centerline portion of thepreform to collapse the hole in the centerline, and the preform wasredrawn to about 8.5 mm diameter 1 meter long void-free Cl-doped silicaglass core canes. Microprobe analysis showed the glass had about 1.8 wt.% Cl and was uniform across the diameter of the canes. Index profile oftheses canes showed about 0.18% delta index (relative to pure silica)uniform across the diameter of the canes.

Example 2

A 1 meter long 3000 gram silica soot preform having a density of about0.5 g/cm³ was prepared in a lathe by flame depositing silica soot onto10 mm diameter removable alumina rotating bait rod comprising a silicahandle. The alumina bait rod was removed creating an open centerlinehole in the preform (which comprised a silica handle), then an 8.5 mmcore cane (comprising a 1.8 wt % Cl-doped silica core from Example 1)was inserted into the centerline hole of the soot preform producing acore-cane soot preform assembly.

The core-cane soot preform assembly was placed into a consolidationfurnace and dried at about 1200° C. in an atmosphere comprising heliumand about 2.5 volume percent chlorine. Following this step, the soot ofthe preform assembly was doped with fluorine for 1 hour in an atmospherecomprising helium and about 1 volume percent SiF₄; then under these flowrates, the assembly was then traversed (down-driven) through a hot zonehaving a peak temperature of about 1500° C. at a temperature ramp rateof about 2.5° C./min to fluorine dope the silica soot and collapse thesilica soot onto the core cane, thus producing a fully densifiedvoid-free glass optical fiber preform having a Cl-doped silica core anda fluorine doped silica cladding.

This preform was placed for about 24 hours in an argon purged holdingoven set at 1000° C. in order to outgas dissolved helium in the glass.The preform was then placed in a redraw furnace set at about 1900° C.,and the preform was redrawn to about 16 mm diameter 1 meter longvoid-free Cl-doped silica glass core F-doped silica clad canes. Indexprofile of theses canes showed about 0.18% delta index core and −0.23%delta index cladding (relative to pure silica).

Example 3

A 1 meter long 16 mm diameter cane from Example 2 was placed on a lathe,then about 3100 grams of silica soot was flame deposited on the caneproducing a cane-overclad preform assembly having an overclad sootdensity of about 0.5 g/cm³. This assembly was placed into aconsolidation furnace and dried at about 1200° C. in an atmospherecomprising helium and about 2.5 volume percent chlorine Following thisstep, the assembly was doped with fluorine for 1 hour in an atmospherecomprising helium and about 1 volume percent SiF₄; then under these flowrates, the assembly was then traversed (down-driven) through a hot zonehaving a peak temperature of about 1500° C. at a temperature ramp rateof about 2.5° C./min to fluorine dope the silica soot and collapse thesilica soot onto the core cane. This produced a fully densifiedvoid-free glass optical fiber preform having a Cl-doped silica core, afluorine-doped silica cladding and fluorine-doped silica overcladding.

This preform was placed for about 24 hours in an argon purged holdingoven set at 1000° C. in order to outgas dissolved helium in the glass.The preform was then placed in a draw furnace and 125 micron diameteroptical fiber was drawn at 15 m/s. The optical fiber had the followingoptical properties: attenuation at 1310 nm, 1550 nm and 1570 nm of0.305, 0.169 and 0.165 dB/km, respectively; mode field diameter at 1310nm and 1550 nm of 8.3 and 9.3 microns, respectively and a 22 meter cablecutoff of 1220 nm.

Optical fibers disclosed herein can be made using the methods describedbelow which utilize preform consolidation conditions which are effectiveto result in a significant amount of chlorine being trapped in the coreregion of the consolidated glass preform. The optical fibers disclosedherein may be manufactured using conventional soot deposition processessuch as the outside vapor deposition (OVD) process or the vapor axialdeposition (VAD) process, in both cases of which silica and doped silicaparticles are pyrogenically generated in a flame and deposited as soot.Alternatively, other processes, such as plasma chemical vapor deposition(PCVD) and modified chemical vapor deposition (MCVD), can also beemployed which can result in the same or higher chlorine levels in theresultant optical fiber preform and thus, the optical fiber that isdrawn therefrom. In the case of OVD, as illustrated in FIG. 5, sootpreform 20 is formed by depositing silica-containing soot 22 onto anoutside of a rotating and translating mandrel or bait rod 24. Thisprocess is known as the OVD or outside vapor deposition process. Mandrel24 is preferably tapered. The soot 22 is formed by providing a glassprecursor 28 in gaseous form to the flame 30 of a burner 26 to oxidizeit. Fuel 32, such as methane (CH4), and combustion supporting gas 34,such as oxygen, are provided to the burner 26 and ignited to form theflame 30. Mass flow controllers, labeled V, meter the appropriateamounts of silica glass precursor 28, fuel 32 and combustion supportinggas 34, all preferably in gaseous form, to the burner 26. The glassformer compounds 28 are oxidized in the flame 30 to form the generallycylindrically-shaped soot region 23.

After forming of the silica soot core preform, as illustrated in FIG. 6,the soot core preform 20 including the cylindrical soot region 23 may bechlorine doped and consolidated in consolidation furnace 29 to form aconsolidated soot core preform. Prior to consolidation, the mandrel 24illustrated in FIG. 5 is removed to form a hollow, cylindrical soot corepreform. During the chlorine doping and consolidation process, the SiO₂soot core preform 20 is suspended, for example, inside a pure quartzmuffle tube 27 of the furnace 29 by a holding mechanism 21. Prior to theconsolidation step the preform 20 is exposed to a chlorine containingatmosphere. For example, a suitable chlorine doping atmosphere mayinclude about 0 percent to 70 percent helium and 30 percent to 100percent chlorine gas, in some embodiments 50 percent to 100 percentchlorine gas, at a temperature of between about 950° C. and 1500° C. anda suitable doping time ranges from about 0.5 and 10 hours.

Using the methods disclosed herein fibers may be made which exhibitchlorine concentrations greater than 1.5 wt % (1.8 mole %), morepreferably greater than 2 wt %, more preferably greater than 2.5 wt %,more preferably greater than 3 wt %, more preferably greater than 3.5 wt%, more preferably greater than 4 wt %, more preferably greater than 4.5wt %, more preferably greater than 5 wt %, which is significantly higherthan chlorine levels utilized previously. Such high chlorine levels canbe achieved by optimizing of the process variables disclosed herein.

For example, higher temperatures may be used to vaporize SiCl₄ liquid,resulting in increased SiCl₄ concentration in the vapor phase. Thevaporizer temperature in some embodiment is higher than 40° C., in someother embodiments is higher than 45° C., some other embodiments ishigher than 50° C. and in yet other embodiments is higher than 57° C. Asa result, increased SiCl₄ concentration may be employed in theconsolidation furnace. In some embodiments, the fraction of the SiCl₄gas through the vaporizer/bubbler to the total flow to the furnace ishigher than 30%, in other embodiments, the fraction of the SiCl₄ gasthrough the vaporizer/bubbler to the total flow to the furnace is higherthan 50% and in yet other embodiments, the fraction of the SiCl₄ gasthrough the vaporizer/bubbler to the total flow to the furnace is higherthan 80%. The remainder of the gas may be helium. In certain otherembodiments, the fraction of the SiCl₄ gas through the vaporizer/bubblerto the total flow to the furnace is 100%, and preferably remains at ashigh a percentage as possible, for example 100%, until sintering of thepreform is complete. In some embodiments, the chlorine doping stepcomprises exposing the preform to a partial pressure of SiCl₄ greaterthan 1 atm. In some other embodiments, the chlorine doping stepcomprises exposing the preform to a partial pressure of SiCl₄ greaterthan 2 atm. In still other embodiments, the chlorine doping stepcomprises exposing the preform to a partial pressure of SiCl₄ greaterthan 3 atm. or greater than 4 atm. In yet still other embodiments, thechlorine doping step comprises exposing the preform to a partialpressure of SiCl₄ greater than 8 atm.

In some embodiments, doping of the preform via exposure to SiCl₄ occursduring the sintering process, i.e. the soot preform is being doped priorto and/or up to the point where the soot preform goes to closed porestate and becomes a fully sintered preform, in presence of SiCl₄ attemperatures higher than 1300° C., in other embodiments at temperatureshigher than 1375° C. In some embodiments the chlorine doping occursduring the sintering process at temperatures higher than 1400° C.

Use of higher soot surface area preforms facilitates higher chlorinedoping levels in the preform on exposure of the preform to SiCl₄. Insome embodiments, the surface area of the soot preform is greater than10 m²/gm; in other embodiments, the surface area of the soot preform ingreater than 20 m²/gm; in yet other embodiments, the surface area of thesoot preform in greater than 25 m²/gm; and in still other embodiments,the surface area of the soot preform in greater than 50 m²/gm. Incertain other embodiments, the surface area of the soot preform ingreater than 90 m²/gm. Surface area of the preform can be measured usingBET surface are characterization techniques.

In some embodiments, the soot preform may also comprise greater than 0.5wt % oxygen depleted silica, i.e. silicon monoxide (SiO). This may beaccomplished, for example, by doping the silica glass soot with SiOpowder or Si. For example, SiO powder may be doped into a SiO₂ sootpreform via soot-pressing and/or doping of the SiO₂ soot preform withSiO vapor in a furnace. In other embodiments the SiO or Si is doped intoa SiO₂ soot preform by introducing some amount of SiH4 to a furnace todecompose SiO₂ to Si or SiO.

The amount of doped SiCl₄ can also be increased by treating silica sootpreform with multiple cycles of successive exposure of SiCl₄ and H2O/O2prior to full consolidation of the preform. Without wishing to be boundby theory, it is believed that the treatment of silica soot surface withSiCl4 results in doping of chlorine by attaching —SiCl3 groups at thelocation of OH groups on silica soot surface (and/or by reacting withthe surface SiOSi groups to form an SiCl+ and SiOCl3). Each of the Clmolecule in attached —SiCl3 group can be converted to OH group bytreating with water (or oxygen to form another SiO2 molecule on thesurface), which then in turn become the reactive sites for attachingadditional —SiCl3 groups on subsequent treatment with SiCl4. Byexploiting the procedure where the preform is exposed to multiple cyclesof successive SiCl4 and H2O (and/or O2) environment, it is possible tocreate a cascading structure and incorporate high amounts of chlorine onsoot particle surface (as an aside, we describe this process as achemistry fractal). This results in significantly higher chlorine dopinglevels in the consolidated glass compared to doped chlorine levelsreported in prior art.

Other methods can be used to increase the soot surface area of thepreform include: 1) low density soot formed using outside vapordeposition 2) pressed high surface area glass soot, and 3) impregnatingthe glass soot with a sol-gel silica (e.g., tetraethylorthosilicate(TEOS), pre or post hydrolyzed) or nano-particle silica such as Ludox®colloidal silica.

Using the methods outlined herein, in some embodiments the dopedchlorine concentration in consolidated glass is higher than 1.5 wt %. Insome other embodiments, the doped chlorine concentration in consolidatedglass is higher than 2 wt %. In yet other embodiments, the dopedchlorine concentration in consolidated glass is higher than 3 wt %.

Gradient sintering may be employed whereby the soot preform 20 is drivendown through a hot zone of the furnace 29 which is maintained at atemperature of between about 1225° C. to 1550° C., more preferablybetween about 1390° C. and 1535° C. For example, the preform may be heldin an isothermal zone which is maintained at a desired chlorine dopingtemperature (950-1250° C.) for a period of time sufficient to enablesufficient doping of chlorine into the preform, after which the sootpreform is driven through a zone which is maintained at a desiredconsolidation temperature (e.g. 1225° C. to 1550° C., more preferably1390° C. and 1535° C.) at a rate of speed which is sufficient to resultin the preform 20 temperature increasing by greater than 1° C./min andat a rate sufficient to form a layer of consolidated glass on theoutside of the preform. In one preferred embodiment, the soot containingpreform is downfed through a consolidation hot zone at a first downfeedrate, followed by downfeeding of the preform through a second hot zoneat a second downfeed rate which is less than that of the first downfeedrate. Such a consolidation technique results in the outside portion ofthe soot preform sintering before the rest of the preform sinters,thereby facilitating trapping of doping gases by the outer consolidatedglass layer, which will in turn facilitate formation of and retaining ofchlorine dopant in the resultant consolidated glass. For example, thepreform can be exposed to such suitable consolidation temperatures (e.g.greater than about 1390° C.) at a first speed which is sufficient toresult in the preform temperature increasing by more than 15° C./min,more preferably greater than 17° C./min, followed by at least a seconddownfeed rate/consolidation temperature combination which is sufficientto result in the preform heating by at least about 12° C./min but lessthan 17° C./min, more preferably greater than 14° C./min but less than15° C./min. Preferably, the first consolidation rate results in theoutside of the preform increasing in temperature at a rate which isgreater than 2, more preferably greater than 3, and most preferablygreater than about 4° C./min higher than the heating rate of the secondconsolidation rate. If desired, a third consolidation step can beemployed which heats at a slower rate (e.g. less than 10° C./min).Alternatively, the soot preform can be sintered at even faster rates bydriving the soot preform through a furnace hot zone where thetemperature is greater than 1550° C., more preferably greater than 1700°C., even more preferably greater than 1900° C. Alternatively, ratherthan doping the soot preform and subsequently forming the consolidatedouter glass layer as described above, the consolidated outer glass layercould be formed prior to the chlorine doping step, and the chlorinedoping can occur via transporting of the dopant gases into the center ofthe preform. Our ability to dope high levels of chlorine providessignificant advantage in making low loss fibers. Chlorine is a dopantthat results in low Rayleigh scattering loss by lowering densityfluctuations contribution, without increasing concentrationfluctuations. In the prior art, optical fibers with cores havingchlorine concentrations less than 1.2 wt % have been disclosed. For suchdesigns, fluorine is used in the overclad to provide the indexdifferential between the core and the cladding. However, because of thelarge viscosity mismatch between the core and the cladding, significantstresses are induced at the draw, which diminish the effectiverefractive index-differential between the core and the inner claddingregion through stress-optic effect and also negatively impactattenuation by impeding the structural relaxation of the glass in theglass transition region. For example, a fiber having 1.1 wt % (1.8 mole%) chlorine in the core and 1.4 wt % (4.4 mole %) fluorine in theadjacent cladding will result in a compositional index differentialbetween the core and the cladding which is 0.505% delta. However, whenthis fiber is drawn at 150 g tension, the effective index differentialin the fiber is greatly diminished due to stress-optic effect and anactual refractive index delta percent of 0.296% delta is achieved. Thisproblem is believe to be due to the core glass being stiffer than thecladding glass, that is the [moles of Cl-core]/[moles F-clad] is lessthan 1.

In the inventive examples presented here, because of the high chlorinelevels in the core, much lower amount of fluorine is needed to obtainthe required index differential for core to act as an effectivewaveguide. Furthermore, the higher level of chlorine doping in the coreand lower fluorine content in the clad also results in better viscositymatching of core and cladding and thereby lower stresses andstress-optic impact. For example, using the techniques described herein,a fiber was manufactured having 1.8 wt % (3.0 mole %) chlorine in thecore and 0.81 wt % (2.5 mole %) fluorine in the cladding thus having aCl_(core)/F_(inner clad) of 2.2 wt. %/wt. % and 1.2 mole %/mole %. Sucha fiber should result in a compositionally based refractive indexdifferential between the core and the cladding which is 0.43% delta.This fiber was drawn at varying tensions of 50, 100 and 150 g, theeffective index differential in the fiber is only slightly reduced dueto stress-optic effect, with the real effective refractive index deltapercent between core and the clad ranging between 0.43% delta and 0.40%delta over the tension range studied. It is also observed that becauseof the reduction in the stresses in the core due to improved viscositymatching, the core-clad index differential obtained in the fiber is alsorather insensitive to the draw tension magnitude. Without wishing to bebound by theory, it is believed that this lower variation betweendifferent draw tensions may be due to the core glass being softer thanthe cladding glass.

After the chlorine doping step, the core soot preform may be sintered.The sintering temperatures employed in the present invention preferablycan range from 1100° C. to 1600° C., more preferably between about 1400and 1550° C., and most preferably between about 1480 and 1550° C. Oneparticularly preferred sintering temperature is approximately 1490° C.Additional information related to manufacturing such void-containingregions within the cladding of the optical fiber can be found, forexample, in U.S. patent application Ser. No. 11/583,098, thespecification of which is hereby incorporated by reference in itsentirety. After sintering the core preform may be drawn to a smallerdiameter and cut into lengths to form consolidated chlorine doped glasscore canes.

Additional soot which will form the inner cladding region may then bedeposited onto the glass core cane using the same method as explainedabove with respect to the core soot deposition process. The innercladding soot can then be doped with fluorine using a dopant gas havingfluorine or other optical fiber dopants therein. For example, SiF₄and/or CF₄ gas may be employed. Such dopant gases may be employed usingconventional doping temperatures, for example between about 950 and1250° C. for 0.25 to 4 hours. The sintering temperatures employed in thepresent invention preferably can range from 1100° C. to 1600° C., morepreferably between about 1400 and 1550° C., and most preferably betweenabout 1480 and 1550° C. One particularly preferred sintering temperatureis approximately 1490° C. Additional information related tomanufacturing such void-containing regions within the cladding of theoptical fiber can be found, for example, in U.S. patent application Ser.No. 11/583,098, the specification of which is hereby incorporated byreference in its entirety.

While the preferred method of making the fibers disclosed herein is viathe outside vapor deposition process, the fibers disclosed herein canalso be prepared, and the same or higher chlorine levels can beobtained, using other techniques such as MCVD and PCVD. For example, acore glass layer may be deposited, via a PCVD (plasma chemical vapordeposition) process, inside a glass tube which is comprised of SiO₂, sothat the core glass layer comprises chlorine doped silica having greaterthan 1.5 wt % chlorine, more preferably greater than 2 wt % chlorine,even more preferably greater than 2.5 wt % chlorine, and even morepreferably greater than 3 wt % Cl. The tube can then be collapsed toeliminate the open centerline and thereby forming an optical fiberpreform. In some embodiments, a fluorine doped cladding layer may beprovided on the fiber preform, either by depositing soot on the outsideof said tube and doping with fluorine, or by starting with a silica tubewhich has already been doped with fluorine. Preferably, as describedabove, the molar ratio of the chlorine in the core portion to thefluorine in the cladding layer is ≥1.

The fibers disclosed herein may be drawn from optical fiber preformsmade using conventional manufacturing techniques and using known fiberdraw methods and apparatus, for example as is disclosed in U.S. Pat.Nos. 7,565,820, 5,410,567, 7,832,675, 6,027,062, the specifications ofwhich is hereby incorporated by reference. In particular, optical fiberis pulled from a root portion of the optical fiber preform by a tractor.After leaving draw furnace, the bare optical fiber encounters a diametermonitor (D) which provides a signal that is used in a feedback controlloop to regulate speed of the tractor to maintain a constant fiberdiameter. The bare optical fiber then passes through a fiber tensionmeasurement device (T) that measures the tension of the optical fibercaused by pulling the fiber from the preform. This tension can increasedepending on the speed of the fiber draw, the temperature and viscosityof the root of the preform, etc. One example of a fiber tensionmeasurement device is disclosed in EP 0479120 A2 which is herebyincorporated herein by reference.

In some embodiments, the optical fibers comprise a primary coatinghaving a Young's modulus of less than 1 MPa and a secondary coatinghaving a Young's modulus of greater than 1200 MPa. In some embodiments,the optical fibers comprise a primary coating having a Young's modulusof less than 0.5 MPa and a secondary coating having a Young's modulus ofgreater than 1500 MPa. In some embodiments urethane acrylate coatingsare employed.

It is to be understood that the foregoing description is exemplary onlyand is intended to provide an overview for the understanding of thenature and character of the fibers which are defined by the claims. Theaccompanying drawings are included to provide a further understanding ofthe preferred embodiments and are incorporated and constitute part ofthis specification. The drawings illustrate various features andembodiments which, together with their description, serve to explain theprincipals and operation. It will become apparent to those skilled inthe art that various modifications to the preferred embodiments asdescribed herein can be made without departing from the spirit or scopeof the appended claims.

What is claimed is:
 1. A method for making an optical fiber preform, themethod comprising: exposing a silica soot core preform having a surfacearea greater than 10 m²/gm to an atmosphere comprising achlorine-containing compound at temperatures higher than 1300° C. toform a chlorine doped soot core preform; sintering the soot core preformto form a core portion of the optical fiber preform such that thechlorine concentration in said core portion is greater than or equal to1.5 wt % chlorine; and forming a cladding layer over said core portionof said fiber preform, said cladding layer comprising fluorine greaterthan 0.15 wt %, wherein a molar ratio of the chorine in the core portionto the fluorine in the cladding layer is ≥1.
 2. The method of claim 1,wherein the soot preform comprises greater than 0.5 wt % siliconmonoxide (SiO).
 3. The method of claim 1, wherein the surface area ofthe silica soot preform is greater than 25 m²/gm.
 4. The method of claim1, wherein the surface area of the silica soot preform is greater than50 m²/gm.
 5. The method of claim 1, wherein the chlorine doping andsintering of the silica soot preform is done in an atmosphere containingSiCl₄ having partial pressure of greater than 0.5 atm.
 6. The method ofclaim 5, wherein the chlorine doping and sintering of the silica sootpreform is done in an atmosphere containing SiCl₄ having partialpressure of greater than 1 atm.
 7. The method of claim 1, wherein thechlorine weight % in the core portion of the optical fiber preform isgreater than 1.5 wt % and the fluorine weight % in the cladding layer ofoptical fiber preform is less than 0.6 wt %.
 8. The method of claim 1,wherein said step of exposing the silica soot preform to an atmospherecomprising a chlorine-containing compound comprises a first chlorinedoping step comprising exposing the preform to an atmosphere comprisingSiCl4, and subsequent to said first exposing step, exposing the preformto atmosphere comprising water or oxygen, and thereafter performing asecond chlorine doping step in an atmosphere comprising SiCl4 attemperatures higher than 1300° C.
 9. The method of claim 1, wherein saidstep of exposing the silica soot preform to an atmosphere comprising achlorine-containing compound comprises a first chlorine doping stepcomprising exposing the preform to an atmosphere comprising 1) a mixtureof SiCl4 and water or 2) a mixture of SiCl4 and oxygen, and said methodfurther comprises chlorine doping the preform in a second chlorinedoping step in an atmosphere comprising SiCl4 at temperatures higherthan 1300° C.
 10. The method of claim 1, wherein the fluorine weight %in the cladding layer is less than 1 wt %.
 11. A method for making anoptical fiber preform, the method comprising: exposing a silica sootcore preform having a surface area greater than 10 m²/gm to anatmosphere comprising a chlorine-containing compound at temperatureshigher than 1300° C. to form a chlorine doped soot core preform; andsintering the soot core preform to form a core portion of the opticalfiber preform such that the chlorine concentration in said core portionis greater than or equal to 1.5 wt % chlorine, wherein said step ofexposing the silica soot preform to an atmosphere comprising achlorine-containing compound comprises a first chlorine doping stepcomprising exposing the preform to an atmosphere comprising SiCl4, andsubsequent to said first exposing step, exposing the preform toatmosphere comprising water or oxygen, and thereafter performing asecond chlorine doping step in an atmosphere comprising SiCl4 attemperatures higher than 1300° C.
 12. A method for making an opticalfiber preform, the method comprising: exposing a silica soot corepreform having a surface area greater than 10 m²/gm to an atmospherecomprising a chlorine-containing compound at temperatures higher than1300° C. to form a chlorine doped soot core preform; and sintering thesoot core preform to form a core portion of the optical fiber preformsuch that the chlorine concentration in said core portion is greaterthan or equal to 1.5 wt % chlorine, wherein said step of exposing thesilica soot preform to an atmosphere comprising a chlorine-containingcompound comprises a first chlorine doping step comprising exposing thepreform to an atmosphere comprising 1) a mixture of SiCl4 and water or2) a mixture of SiCl4 and oxygen, and said method further compriseschlorine doping the preform in a second chlorine doping step in anatmosphere comprising SiCl4 at temperatures higher than 1300° C.